This invention is directed to a surface-treated aluminum material which may be used in such application as automobiles, electric appliances, cans, and the like, wherein the aluminum materials are inevitably subjected to press-working and spot resistance welding. More particularly, this invention is directed to a surface-treated aluminum material which is provided with remarkably improved workability, spot weldability, and corrosion resistance.
Recently, it has become an emergent issue to reduce weight of automobile chassis to thereby improve fuel efficiency and reduce CO.sub.2 exhaustion, as strongly appreciated in view of global environment, and in particular, in view of preventing warming of the earth. Under such conditions, use of an aluminum alloy for automobile chassis have attracted a considerable attention because of reliability and workability, as well as reusability of the material. Use of an aluminum alloy plate for automobile chassis, therefore, has only recently been started.
Aluminum materials are also used for electric appliances, office automation equipments, building materials, can materials, and other applications. In the production of such products, aluminum materials are also subjected to frequent press-working as well as spot welding.
Aluminum materials are markedly inferior to steel materials in their spot resistance weldability, in particular, with regard to the life of the welding electrode employed, inevitably resulting in a reduced productivity of the final products including the automobile chassis, whose production may involve repeated spot welding.
More illustratively, spot welding of an aluminum or an aluminum alloy plate is likely to be accompanied with deposition of molten aluminum on the electrode. Furthermore, the number of spot welds with an adequate nugget diameter that could be made with one electrode is significantly small compared to the spot welding of steel plates. Therefore, a frequent dressing or replacement of the welding electrode was required to prevent such deposition of the molten aluminum on the electrode and excessive reduction in the nugget diameter, and such a frequent dressing or replacement of the electrode adversely affected the welding efficiency of the aluminum or aluminum alloy plates, and hence, productivity of the final product.
Since it is only of comparatively recent date that an aluminum or an aluminum alloy plate is utilized in an industrial scale production wherein spot resistance welding is involved, there are only few adequate proposals for overcoming the above-described problems. Japanese Patent Application Kokai No. 60(1985)-187483 proposes an improvement by adequately controlling the thickness of the oxide film on the welding surface of the aluminum plate by means of an anodic treatment. Japanese Patent Application Kokai Nos. 53(1978)-6252, 53(1978)-48954 and 53(1978)-48955 disclose an improvement by disposing a zinc, titanium or stainless steel thin film between the welding surfaces of the aluminum or aluminum alloy plates; and Japanese Patent Application Kokai No. 57(1983)-4389 discloses an improvement by providing a chromium, nickel, copper or silver-plated layer on the aluminum or aluminum alloy plate; although these are not specifically developed for the aluminum alloy plates adapted for automobile uses.
Of the above-described proposals, the anodic treatment for controlling the oxide film thickness is difficult to consistently carry out, rendering the process unreliable and unpractical. The provision of the zinc, titanium or stainless steel thin film between the welding surfaces of the aluminum alloy plates is inadequate for the aluminum alloy plates for automobile use. The provision of the chromium, nickel, copper or silver-plated layer on the aluminum alloy plate is unpractical since the adhesion of the plated layer to the aluminum alloy substrate is insufficient.
Compared to steel materials, an aluminum or an aluminum alloy is soft and poor in tensile properties, and therefore, die galling is frequently induced in the press-working of such a material.
In press-working of the aluminum or aluminum alloy plates, a lubricating oil is usually applied on the surface of the plate. Such application of the lubricating oil, however, suffered from various problems including:
(1) die galling in the press-working could not be prevented by simply applying the lubricating oil; PA1 (2) the lubricating oil is usually applied by spraying, which resulted in spreading of the lubricating oil to adversely affect the working conditions; and PA1 (3) degreasing/washing step after the press-working was inevitable before the product could be assembled with other products, and such degreasing/washing involved use of a hazardous solvent such as a chlorofluorocarbon or 1-1-1-trichloroethane which adversely affected the working conditions and arose environmental problems. PA1 (A) an aluminum or aluminum alloy substrate; PA1 (B) a chromate film deposited on the substrate to a coating weight of from 5 to 100 mg/m.sup.2 calculated in terms of elementary chromium on the first major surface; and PA1 (C) an organic resin mixture film deposited on the chromate film to a dry film thickness of from 0.05 to 3.0 .mu.m, said organic resin mixture film comprising PA1 (A) an aluminum or aluminum alloy substrate; PA1 (B) a chromate film deposited on the substrate to a coating weight of from 5 to 100 mg/m.sup.2 calculated in terms of elementary chromium on the first major surface; and PA1 (C) an organic resin mixture film deposited on the chromate film to a dry film thickness of from 0.05 to 3.0 .mu.m, said organic resin mixture film comprising PA1 (A) an aluminum or aluminum alloy substrate; PA1 (B) a chromate film deposited on the substrate to a coating weight of from 5 to 100 mg/m.sup.2 calculated in terms of elementary chromium on the first major surface; and PA1 (C) an organic resin mixture film deposited on the chromate film to a dry film thickness of from 0.05 to 3.0 .mu.m, said organic resin mixture film comprising PA1 a surface-treated aluminum material having improved spot resistance weldability and workability comprising PA1 a surface-treated aluminum material having improved spot resistance weldability and workability with no lubricant oil comprising PA1 a surface-treated aluminum material having improved spot resistance weldability, workability and corrosion resistance comprising PA1 wherein the aluminum substrate has one surface on which welding electrode is to be disposed upon spot welding and the other surface which is to be welded with another material, and the chromate layer (B) is deposited, or optionally is not deposited, on the surface on which the welding electrode is to be disposed to a coating weight equal to or smaller than 3/4 of the coating weight of the chromate layer (B) on the surface to be welded.
Therefore, there is a demand for an aluminum plate material which could be press-worked without application of a lubricating oil and which does not require degreasing step after the press-working of the material. Japanese Patent Application No. 2(1990)-310036 and 3(1991)-180218 disclose a wax-coated aluminum plate, which has been developed as a can material. This material, however, suffers from difficulty in the control of the coating weight of the wax. When the coating weight of the wax is excessively small, this material exhibits poor workability, while an excessively large coating weight of the wax results in contamination of the mold. Therefore, such a can material could only be used for limited applications. This material also suffers from an insufficient corrosion resistance when the material is used without any post-treatments, since the surface of the aluminum plate would become exposed after the press-working of the material. Such an application of the solid lubricant also results in a reduced spot weldability, which is another serious problem.
It was quite difficult to achieve stable effects under different working conditions by such conventional process wherein the workability is improved by the wax application. Furthermore, such conventional process required an anti-corrosion treatment such as coating after the press-working to prevent rust from developing under some conditions. Therefore, it was quite difficult to omit the washing step after the press-working or use of the lubricating oil in the press-working.
Although an aluminum or an aluminum alloy plate is inherently provided with an excellent corrosion resistance, it was the oxide film formed on the surface of the plate which was responsible for such an excellent corrosion resistance. Therefore, when an aluminum or an aluminum alloy plate is subjected to spot welding or press-working and crevices or hems are formed wherein no such oxide film is present, a so-called crevice corrosion is likely to be induced in such hems and crevices.